Ion source having a high work function material coating the outer surface of the ionizer



y 1968 P. ZALM 3,382,397

ION SOURCE HAVING A HIGH WORK FUNCTION MATERIAL COATING THE OUTER SURFACE OF THE IONIZER Filed Nov. 12. 1964 INVENTOR.

Pl ETE R ZAL M AGEN 3,382,397 ION SOURCE HAVING A HIGH WORK FUNCTIGN MATERIAL COATING THE OUTER SURFACE OF THE IONIZER Pieter Zalm, Emmasingel, Eindhoven, Netherlands, as-

signor to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Nov. 12, 1964, Ser. No. 410,584 Claims priority, application Netherlands, Nov. 20, 1963, 300,777; Feb. 12, 1964, 64--1,192 4 Claims. (Cl. 313-230) ABSTRACT 015 THE DESCLGMIRE An ion source comprising a supply of an ionizable material behind a porous tungsten substrate having on its outer surface a coating of iridium, ruthenium, rhenium, or osmium.

This invention relates to ion sources in which ions of alkali or alkaline earth metals or other low ionization potential substances are produced on a heated anode comprising a porous refractory metallic body which forms the closure for a supply of alkali or alkaline earth metal or its compound. One form of such a device useful for ion propulsion engines is described in a copending U.S. application Ser. No. 363,580, nOW abandoned, filed Apr. 29, 1964, and U.S. Patent No. 3,141,769.

The efficiency of such an ionic source, that is to say the ratio between the number of ionized atoms and the total number of atoms leaving the porous body, is the higher as the work function of the porous metal is higher or, in other words, the difference between said work function and the ionization potential of the alkali or alkaline earth metal is greater. Hitherto, tungsten has been used for the porous metal.

In accordance with my invention, I obtain an improvement in such devices by constructing the porous refractory metal body which forms the closure of tungsten superficially covered with a thin coating of one of the metals iridium, ruthenium, rhenium, or osmium. The coating need be only from about 100 to 5,000 Angstrom units thick. Since the coating metal has a work function considerably higher than that of tungsten, the efiiciency of my ion source is higher than in the case where the coating is omitted from the tungsten body. On the other hand, only a very small amount of the coating material, which is very expensive, need be used, so that the cost thereof is minimal. The coating may be applied by various means, such as by electrolytic means or by cathode sputtering or by spraying as described in the abovementioned copending application.

The invention will now be described in greater detail with reference to the accompanying diagrammatic drawing of which the sole figure is a cross-sectional view of one form of ion source according to my invention.

The figure shows an ion source according to my invention comprising a vacuum-tight envelope 1 which houses a molybdenum body or support 2 having, on the one hand, a space for a heating filament 3, and, on the other hand, a space for a supply 4 of rubidium compounds or other low ionization potential metal or substance. Th supply 4 is closed by a densely-sintered porous tungsten body 5, covered with a thin layer 6 of rhenium. By means of an electrode 7 made negative relative to the body by means not shown, the ions generated on the surface 6 are accelerated towards an ion-optical system 8 and subsequently collected in an absorptive layer 9 on a collector 10. Instead of rubidium compounds, as explained, it is possible to use other alkali or alkaline earth metals. In addition,

} United States Patent 0 3,382,397 Patented May '7, 1968 the collector may be omitted and the envelope may be open, thus communicating with a further vacuum space in which the ions are used. Also, the ion source may form part of an ion propulsion engine. Instead of rhenium, iridium ruthenium and osmium may be used.

The thin, high work function metal, coating 6 is porous, as is required to enable the rubidium vapor and ions to flow through. When the coating is applied electrolytically or by cathode sputtering, by techniques well known in the art, in the thicknesses of about to 5000 Angstroms, the coating is inherently porous. If applied by spraying a slurry of fine powders dispersed in a suitable vehicle, thicker coatings which will also be porous can easily be obtained. After spraying, the powder coat is desirably sintered to the tungsten substrate.

My experiments have demonstrated a vaporization efliciency of lithium on a thin coating of iridium of about 20% at a temperature of about 1400 C., which is in good agreement with the Langmuir-Saha relation.

It may be noted here that in the known ion sources with a tungsten emitter only ions of caesium (ionization voltage 3.87 v.) could be formed with good efliciency. The difference between the Work function of tungsten 4.3 v. and the ionization potential of rubidium 4.16 v. is already too small for a good efficiency. Potassium, sodium and lithium ions could only be formed on those crystal faces of tungsten on which the highest work functions obtain, because the work function for polycrystalline tungsten is 4.3 v., which is lower than the ionization voltages for the three alkali metals given viz. 4.32, 5.12 and 5.36 v. As for the metals osmium, iridium, ruthenium and rheniurn the work functions for polycrystalline material vary from 5.1 to 5.6 v., thus most alkali metals will be ionized on them with good efficiency.

What is claimed is:

1. An ion source comprising a source of material exhibiting a low ionization potential, a porous refractory metal wall member closing off said source for ionizing the low ionization potential material which passes through its pores, and means for heating said porous member at a temperature at which electrons are stripped from thelow ionization potential material, said porous member comprising a preformed porous tungsten substrate with a porous coating deposited on its surface remote from the low ionization potential material source of a refractory metal having a higher work function than tungsten and selected from the group consisting of iridium, rhenium, ruthenium, and osmium, whereby said ion source exhibits a higher efiiciency than the comparable source without the deposited high work function metal coating.

2. An ion source as claimed in claim 1 wherein the substrate comprises a densely sintered, porous tungsten body.

3. An ion source as set forth in claim 1 wherein the coating has a thickness of about 100 to 5000 Angstroms.

4. An ion source as set forth in claim 1 wherein the coating is thin and is vapor-deposited, and the low ionization potential material is selected from the group consisting of alkali and alkaline earth metals.

References Cited UNITED STATES PATENTS 2,754,442 7/1956 Boutry et al. 3l3--63 2,777,086 1/1957 Lederer 313-346 X 3,119,232 1/1964 Richley et a1 31363 X JAMES W. LAWRENCE, Primary Examiner. S. A. SCHNEEBERGER, Assistant Examiner. 

